1. Field of the Invention
The present invention relates to a method and apparatus for high throughput thin film deposition upon and substrate handling of optical disk substrates. In particular, multi-layer thin films are deposited on substrates which are subsequently finished for use in optical disk applications such as DVD (digital versatile disk). To obtain increased processing efficiencies, the deposition apparatus includes a moving web which holds substrates in position as they are carried from one processing location to another.
2. Brief Description of Background Art
Throughput or processing rate (number of substrates processed per hour) directly impacts on the cost of manufacturing the optical disks. Due to the nature of the process steps used to produce an optical disk, and particularly thin film deposition where vacuum maintenance requirements and cleanliness (avoidance of substrate contamination by particulates) is critical in ensuring film quality, the optical disk industry has struggled with system-related throughput problems.
One of the more popular methods of optical disk production used to improve throughput is the cluster tool. The cluster tool is a multi-chamber tool sharing a common substrate handler. From a cost perspective, multiple chambers running the same process in parallel and sharing a common substrate handler can provide throughput which is superior to an equivalent number of stand-alone single substrate tools or to an in-line system (which will be described subsequently herein). A multi-chamber tool uses floor space and processing time more efficiently than a number of stand-alone single substrate tools. The substrate handler also has much less idle time than if it were only servicing one chamber. While one chamber is busy, the handler can still transfer wafers to or from other pods of the multi-chamber system.
Additional economic benefits accrue when a series of processes that could be performed separately are linked in one tool. The reduced substrate handling and decreased numbers of pump-and-vent cycles decrease foreign material, especially in vacuum-equipped clusters used for plasma processing. Eliminating substrate transfer from tool to tool reduces the substrate processing time through the cluster and decreases the cycle time and lot turn-around time. When substrates are processed in a continuous manner, delay times between sequential steps can be more tightly controlled. Clusters that keep substrates under vacuum during diverse sequential process steps allow new processing options since surface interactions with atmosphere and moisture are avoided. Multiple repetitive steps are also more attractive. Despite these advantages, the types and capabilities of the component process modules/chambers and substrate transfer capabilities limit the flexibility of the cluster tools on the market. One of the most significant detractors to cluster processing is mean time-to-failure (MTTF); this must be addressed with high priority, especially since an entire cluster is incapacitated when a single module fails.
The overall control of the multi-layer processing may be the most difficult aspect of the cluster system. Due to an intrinsic nature of substrate routing sequence in a cluster tool system, the presently available software limits the maximum throughput. The most popular cluster tool""s throughput is limited to about 72 substrates per hour for a single-layer deposition. When three different layers must be processes, the presently available throughput is 36 to 40 substrates per hour in a 3-chamber/module system.
The production system most commonly used in the industry employs an in-line batch system for the thin film deposition process. In in-line batch systems, the substrates are loaded onto a substrate holder or pallet. In a batch sputter-down system, the substrates may only be placed on a holder. Under this configuration, there are defects generated from the sputter target and shield areas, which affect on film quality and consequently product yield. Use of pallets also increases the particle occurrence and overall cost of ownership (COO) from maintenance and spare stocks.
An in-line batch system may be able to handle up to about 100 substrates per hour for a single layer film deposition; however, because of batch-nature of the substrate loading, there is a penalty for throughput loss from engineering reliability such as loading and unloading steps. In addition, there is a build up of particulates on the moving carrier used to move the substrates through the deposition processing area. For example, a moving belt which repeatedly circles within the process area, handling a series of substrates, accumulates particulates and requires a cleaning process for particulate removal, leading to high maintenance downtime. In addition to maintenance downtime, particulates which accumulate on the belt may contaminate substrates which contact the belt. Belt hardening occurs over time, requiring belt replacement. When a more complicated product is produced, such as one which requires a three layer film deposition, the slowest deposition process dictates the overall throughput rate for the system, since such systems are not equipped with flexible hardware of the kind which will be described with reference to applicant""s invention. In addition, presently known in-line batch systems frequently experience cross-contamination when more than one film material is deposited, due to lack of proper isolation shields between material deposition areas.
Web deposition systems have long been used in the preparation of coated substrates such as metallized films. As its name implies, web deposition or coating involves the vacuum deposition of thin films onto flexible substrates such as films which act as moving webs. The substrate is unrolled from a feed reel at the beginning of the web, the deposition is made on the substrate surface, and then the substrate is rolled back up again on a take up reel. The deposition rate and the film thickness required limit the speed at which the substrate travels past a deposition station. Web coaters often contain several deposition stations that coat the substrate sequentially as it moves past them.
Vacuum web coating is the expansion of vacuum coating to large-surface, web-shaped substrates. The substrate is coated in a partial vacuum as the substrate passes by the deposition source along the path between the feed reel and the take up reel. An interesting history of web coating is provided in an article by E. O. Dietrich et al., entitled xe2x80x9cVacuum Web Coatingxe2x80x94An Old Technology With A High Potential For The Futurexe2x80x9d, Society of Vacuum Coaters, 40th Annual Technical Conference Proceedings, pp. 354-364 (1997). These systems involve the application of coatings to a substrate which is moved past the coating source in the form of a continuous web which is unwound from one roll and rewound upon another roll.
In addition to sputter or evaporated coatings, plasma assisted chemical vapor deposition films have been deposited upon moving web substrates. Web coaters may contain several deposition stations that coat the substrate sequentially as it moves past them. Some of the more significant problem areas include the release of water vapor from the substrate during web coating; buckling or tensioning along the longitudinal edges of the web; transverse warping of the web of substrate material; and the presence of particulate contaminants. Transverse warping of the web is caused by the force of gravity acting upon the web, the elongated path of travel which the web of substrate material follows, stresses from external sources developed upon the web of substrate material, the high deposition temperatures to which the web of substrate material is continuously subjected, and the forces created by the highly stressed semiconductor alloy material deposited upon the web of substrate material. U.S. Pat. No. 4,664,951 to Joachim Doehler, issued May 12, 1987 describes a method of providing for corrective lateral movement of a web of substrate material which is adapted to continuously move in a longitudinal direction through a vapor deposition processor.
In an article entitled xe2x80x9cErasable Phase-Change Optical Materials, MRS Bulletin, September 1996, pp. 48-50, Noboru Yamada describes various materials of the kind which can be used to form an erasable optical disk. The erasability is based on optical memory materials which undergo phase changes affecting optical transmission. In practical systems, a laser beam focused into a diffraction-limited spot is used for recording. This enables the spacial size of one bit of data to be very small (of submicron order) so that the recording density is very high. Amorphous recording marks are formed in crystallized areas along tracks. The mark size is about 0.5 xcexcm. The phase-change optical-memory materials must have proper optical constants; an absorption edge that shifts in the visible or near-infrared wavelength region with phase transitions; a suitable melting pointxe2x80x94the materials must be able to be melted with an available laser power, but must not melt at such a low temperature that self-crystallization occurs; and, there must be a rapid and stable phase-transition process. The number of materials meeting this requirement are limited.
The Yamada article describes an example erasable optical disk sample where the substrate is PMMA (polymethyl methacrylate) having deposited on its surface three layers. A first layer of ZnS (dielectric undercoat) a second GExe2x80x94Sbxe2x80x94Te active layer, and an undefined overcoating layer. After deposition of the three layers on the PMMA substrate, an overlying PMMA substrate layer is applied using a photopolymer adhesive. The Yamada article does not describe the apparatus used to fabricate the erasable optical disk; however, it is readily apparent to one skilled in the art that stresses introduced into the depositing materials are likely to have an effect on crystallization and phase-transition processes. With this in mind, it would be highly desirable to have a high throughput apparatus which does not introduce stress into the substrate material or into the thin films being deposited on that material.
In accordance with the present invention, an apparatus and method are described which provide high volume, high throughput deposition of thin films on substrates useful in electronic applications such as optical disks, chip-scale packaging, and plastic based display, by way of example, and not by way of limitation.
An apparatus commonly includes: (a) a continuously moving web for simultaneously transporting a number of substrates to which a thin film of material is to be applied, wherein said moving web is commonly a roll-to-roll moving web; (b) a central processing chamber which is maintained under vacuum and through which at least a portion of said continuously moving web travels; and, (c) at least one deposition device which is located within said central processing chamber, where at least a portion of said continuously moving web is exposed to material deposited from said deposition device. Typically the apparatus also includes: (d) a first moving platform which transfers a substrate onto said continuously moving web; and (e) a second moving platform which receives processed substrates from said continuously moving web.
In one method of depositing at least one thin film on a substrate useful in electronic applications, comprises the steps of: placing a series of substrates onto a continuously moving disposable web; exposing a surface of the moving disposable web on which the substrates are setting to at least one depositing material, to form at least one layer of material on a substrate; and, unloading the substrate from the continuously moving disposable web. Typically the depositing material is sputter deposited. Preferably, sputtering is carried out using a planar magnetron, and wherein the RF power applied to a sputtering target is about 100 to about 5,000 W at a frequency of about 10 to about 30 MHZ. Typically the pressure at the surface of the substrate during deposition of a material is a vacuum of about 10xe2x88x925 torr (1.3xc3x9710xe2x88x923 Pa) or an increased vacuum, whether material deposition is by sputtering or another technique. The preferred moving disposable web is a roll-to-roll web, and the roll speed of the web is based on a required film thickness of a depositing material layer which has a narrow processing window. Substrates are held in place on the moving disposable web by friction or electrostatic attraction.
When only one thin film layer is to be deposited on the substrate, one deposition device is typically adequate, depending on the thickness of the film to be deposited. When multiple overlying thin film layers are to be produced, generally the number of layers determines the number of deposition devices. Preferably the at least one deposition device provides for sputtering or reactive sputtering of material to produce a thin film layer on the substrate, although evaporation and chemical vapor deposition and other coating deposition techniques may be used as well. Typically, in the production of optical disks, the number of deposited thin film layers of different materials is in the range of 4 or fewer. When the thin film layers are deposited using sputtering techniques, this may require the use of up to four different sputtering targets. The target may be attached to or may be the cathode of the sputtering device.
To simplify operational requirements, the power to a sputtering device cathode is independently on without interruption, enabling a continuous operation until shutdown is required for purposes of repairing or replacing the continuously moving web. A preferred method for moving the web is continuous transfer of a web material from one roll to another; a roll-to-roll moving web. Rolls of material used to supply the web may be spliced together to permit continuous operation of the web. For purposes of simplification and reliability, the preferred power to the cathode targets for sputtering is by radio frequency (RF) for both ceramic and metal target materials.
An advantageous sputtering source for each cathode is a planar magnetron. The use of a rectangular (rather than circular) shaped sputtering target in combination with the sputtering source enables the deposition of a high quality film at reduced target cost. For example, a sputtering target about 120 mm wide by 300 mm long is adequate when two optical disks transverse the continuously moving web and travel side-by-side along the longitudinal distance of travel on the web. A wider web, permitting processing on an increased number of optical disks positioned transversely across the width of the web would require a wider sputtering target or more than one target adjacent to each other and in a transverse direction relative to the moving web. A sputtering target may be angled to the substrate plane to provide improved step coverage. Preferably, the angle is less than 20 degrees from horizontal to the plane of the continuously moving web; typically the angle is about 10 degrees. An optimum deposition rate may be obtained by changing the distance between the sputtering target surface and the substrate and by changing the power to the sputtering target, or by using a combination of these techniques.
When more than one sputtering target is used (in a longitudinal direction relative to the moving web), so that different coating materials may be applied in overlying layers, a shield between neighboring sputtering targets is typically required to isolate RF interference between neighboring targets (cathodes). A shield also function as a barrier to cross-contamination between deposition areas for different materials. In one embodiment, the shield is xe2x80x9cTxe2x80x9d shaped, with the arms of the T at the base of the shield, near the substrate surface, so the arms catch falling debris and prevent the debris from contaminating the substrate as it passes by.
Preferably, the first moving platform for loading the substrates onto the moving web (the loading platform) is positioned adjacent the moving web and within the central processing chamber. Or, the first moving platform may reside in a loading chamber (loading plenum chamber) which leads to the central processing chamber. One skilled in the art can envision a number of different possibilities for loading the substrates onto a moving web which carries them by various deposition stations. The loading platform includes apparatus elements, preferably a table supporting a moving belt, which can be made to move in an x-direction or in a y-direction, where x-represents the longitudinal direction of the moving web which carries the substrates and y-represents the transverse direction to the moving web. The x- and y-directional movements enable an incoming substrate to be rotated and translated for positioning on the moving web. Optionally, the first moving platform may include a hand-off pusher which engages the substrate with the moving web.
Preferably, the second moving platform, which is used for moving the substrates off the moving web (the unloading platform) is also located within the central processing chamber, for purposes of minimizing cross contamination during unloading of the substrates. Or, the unloading platform may be located in an unloading chamber (unloading plenum chamber) which leads away from the central processing chamber. Preferably, the second moving platform is also capable of performing x- and y-directional movements (similar to one for loading in the entrance), for purposes of dis-engaging substrates from the continuously moving web. Optionally, a push or kick device may be used to clear substrates from the moving web and make clear the path for exiting substrates arriving at the unloading platform.
The first and second moving platforms may be timed or set to provide x- and y- directional motions which correspond with the speed of the moving web. A position sensor may be placed in the area of the second moving platform so that substrate positions can be adjusted relative to a storage container which is used to collect the substrates as they exit the central chamber or the unloading plenum chamber. central processing chamber which is maintained under vacuum by a dedicated high vacuum system, permits application of a base vacuum of at least 10xe2x88x926 torr (1.3xc3x9710xe2x88x924 Pa). The central processing chamber is not exposed to air under normal deposition and substrate loading and unloading steps and is typically operated at a vacuum of at least 10xe2x88x924 torr (1.3xc3x9710xe2x88x922 Pa).
Typically a plenum chamber (loading plenum chamber) is used Co bridge between a load-lock area in which substrates entering the processing system are placed and the central processing chamber. Another plenum chamber (unloading plenum chamber) is used to bridge between the central processing chamber and a load-lock area for unloading processed substrates. The plenum chambers are under a moderate vacuum which is commonly in the range of about 10xe2x88x924 torr (1.3xc3x9710xe2x88x922 Pa), which is pumped down by a second dedicated vacuum pump. Substrates are continuously transferred via vacuum isolation valves which operate between the load-lock areas and the plenum chambers and between the plenum chambers and the central processing chamber. The vacuum isolation valves repeatedly open and close as substrates are passing into and out of each chamber. Process system control software maintains the relative positioning of opening and closing valves so that the isolation valve between a load-lock area and a plenum chamber is not open at the same time as the isolation valve between the plenum chamber and the central processing chamber, to prevent exposure of the central processing chamber to an adverse vacuum condition or xe2x80x9cvacuum backxe2x80x9d.
Preferably the continuously moving web is made of a polymeric material; and, a preferred polymeric material, not by way of limitation is PET (polyethylene terephthalate).
Typically, the apparatus also includes a cooling device which permits the cooling of the continuously moving web and substrate if necessary. A variety of different cooling methods are applicable. A particularly useful cooling device is a cooling channel is embedded inside a roll drum. A roll drum refers to the very slight curvature of the web surface in the longitudinal direction between the web feed roll and the web take-off roll, to provide for proper web tension and movement control. The speed of the web is preferably determined by the speed of the take-off roll.
Process variable control is implemented using a computerized control system which is programmed to account for factors such as web speed; speed of the loading platform; speed of the unloading platform; loading and unloading cassette elevator speed; power applied to a sputtering target and adjustment of the sputtering process or reactive sputtering process, to provide for balanced and optimal thickness depositions for a particular product; pump speed of a vacuum pump used to create vacuum within a process chamber; power to a heating source or cooling source; traveling web replacement; system maintenance such as shield cleaning; and/or similar process variables. The programming of the control system provides coordination between at least a portion of the process variables, such as synchronization of isolation (slit) valves with the web speed and a substrate position.